In a continuous casting method of molten metal, a casting mold having a casting space for forming a cast slab surrounded at four sides by water-cooled copper plates is used, molten metal is injected into the casting mold, the part of the molten metal contacting the casting mold solidifies to form a shell, the shell is pulled out from the bottom of the casting mold while growing, and the metal finally finishes solidifying whereupon a continuously cast slab is formed.
In continuous casting of a cast slab of a flat shape, the casting space in the casting mold also has a rectangular cross-section. The surfaces of the casting mold facing the long sides of the cross-sectional rectangular shape are called the “long side surfaces” while the surfaces of the casting mold facing the short sides of the rectangular shape are called the “short side surfaces”. The molten metal is supplied through a submerged entry nozzle into the casting mold. The submerged entry nozzle is a closed bottom cylindrical shape. Near the bottom end of the submerged entry nozzle, discharge ports are formed oriented in two directions in the longitudinal direction of the casting space. The discharge ports discharge molten metal inside the casting mold. The discharge flow from the discharge ports of the submerged entry nozzle penetrates in the molten metal pool in the casting mold and strikes the casting mold short sides whereupon it is divided in an upward oriented flow and a downward oriented flow.
At the surface of the molten metal pool formed in the casting mold, continuous casting mold flux is supplied forming a layer. This is melted by the heat of the molten metal and flows into the gap between the casting mold and the shell to form a mold flux film there. This functions as a lubricant between the casting mold and shell. The casting mold constantly vibrates in the vertical direction (called “oscillation”) to promote the inflow of the mold flux film and facilitate withdrawal of the cast slab. On the other hand, the cast slab surface is formed with relief shapes called “oscillation marks” due to the casting mold oscillation.
If arranging an electromagnetic coil around the casting mold having a current path surrounding the casting space and running an alternating current through this electromagnetic coil, a pinch force acts on the molten metal in the casting mold. Japanese Patent Publication (A) No. 52-32824 describes an invention making this electromagnetic force act near the meniscus of the molten metal and thereby causing the molten metal near the meniscus in the casting mold to receive force in a direction separating it from the casting mold wall and making the meniscus strongly bend and simultaneously enlarging the gap between the casting mold and the shell to thereby promote the inflow of powder, reduce oscillation marks, and improve the shape of the cast slab surface.
On the other hand, the thus acting electromagnetic force simultaneously forms an electromagnetic ally driven flow at the molten metal pool in the casting mold. The electromagnetic ally driven flow is formed at the center of the electromagnetic coil in the height direction heading from the shell to the center of the molten metal pool and is divided into the upward oriented flow and the downward oriented flow at the pool center. At a location corresponding to the top half of the electromagnetic coil, a circulating flow is formed comprised of an upward oriented flow at the pool center, an outwardly oriented flow at the meniscus part, and a downward oriented flow near the shell. At a location corresponding to the bottom half of the electromagnetic coil, a rotary flow is formed comprised of a downward oriented flow at the pool center, an outwardly oriented flow near the bottom end of the electromagnetic coil, and an upward oriented flow near the shell.
Japanese Patent Publication (A) No. 11-188460 describes, in an example of casting a billet having a circular or rectangular casting cross-section, a method of continuous casting arranging a molten metal injection nozzle having discharge ports opening in a downward oriented direction so that the discharge ports are positioned below the center of the electromagnetic coil and injecting molten metal into the casting mold from the discharge ports of the molten metal injection nozzle. In what is described in Japanese Patent Publication (A) No. 11-188460, due to this, the rotary flow flowing upward oriented at the center of the molten metal pool is not affected by the discharge flow from the molten metal injection nozzle, so it is considered that a cast slab superior in surface properties is cast.
The molten metal refined by oxygen for decarburization at a refining furnace contains free oxygen, so when transferring molten metal from the refining furnace to a ladle, a deoxidizing agent with a strong deoxidizing power is added into the molten metal to convert the free oxygen to oxides. The nonmetallic oxides formed mostly float up in the molten metal to be separated, but part remains floating in the molten metal and are transferred as is to the tundish. For this reason, the molten metal supplied from the tundish through the immersion nozzle to the inside of the casting mold includes nonmetallic inclusions. Further, to prevent the nonmetallic inclusions in the molten metal from sticking to the inside walls of the submerged entry nozzle, nonoxidizing gas is blown in the submerged entry nozzle. The blown nonoxidizing gas is entrained in the molten metal to become bubbles which move together with the molten metal. These nonmetallic inclusions and bubbles in the molten metal are supplied from the discharge ports of the submerged entry nozzle together with the discharge flow to the inside of the casting mold. If the nonmetallic inclusions and bubbles are entrained in the cast slab, they form quality defects, so it is preferable as much as possible to make them float up in the molten metal in the casting mold and have them absorbed by the continuous casting mold flux covering the meniscus for separation.
In recent continuous casting, the mold is made a vertical bending type provided with a vertical part directly under the meniscus to promote the flotation and separation of the nonmetallic inclusions and bubbles at the vertical part. Further, if the discharge flow from the discharge ports of the submerged entry nozzle strikes the casting mold short sides, then flows downward along the casting mold short sides too strongly, the nonmetallic inclusions and bubbles riding this flow will reach the deep parts of the cast slab and be entrained in the solidified cast slab.